The present invention relates generally to electronic controls and more particularly to fast-response feedback-stabilized control circuits.
A control circuit such as a direct current amplifier receives an input signal and amplifies it to provide an output signal which is used, for example, to regulate a physical process by means of an electromechanical transducer. Such an amplifier may take the form of a differential amplifier which provides an output signal having a magnitude determined by any difference between a first input signal indicative of a desired state of the process and a second input signal indicative of the actual state of the process. In response to the output signal, the transducer regulates the process so as to cause the actual state of the process to approach the desired state.
It is sometimes required that the physical process commence precisely at a certain moment, for example to enable the duration of the process to be accurately controlled or to synchronize the commencement of the process with an external event. Under such circumstances an enabling input signal, which may be generated either manually or by automatic means such as a computer, is used to start and stop the process. When it is desired that the process commence, this enabling input signal is applied to the amplifier and causes the amplifier to provide the output signal.
An example of a physical process which is controlled in the above-described manner is the rate of mass flow of a fluid. A fluid mass flow controller has a sensor which measures the rate of mass flow of a fluid and provides a "mass flow" signal indicative of the measured rate of mass flow. An externally-generated "set point" signal indicates a desired rate of mass flow. When an enabling input signal indicates that the fluid should start flowing, a differential amplifier provides an output signal indicative of any difference between the mass flow and set point signals. An electromechanical transducer--specifically, a solenoid valve--opens in response to the valve signal to permit the fluid to begin flowing. The valve opens more or less according to the magnitude of the valve signal to increase or decrease the rate of mass flow of the fluid so as to minimize any difference between the mass flow and set point signals and thereby cause the actual rate of mass flow to approach the desired rate.
A feedback network having reactive components such as capacitors is commonly used in conjunction with a direct current amplifier, the network and the amplifier together defining a control loop, for example to prevent oscillation, to improve stability, to provide a desired input impedance, or the like. However, these reactive components are characterized by a time constant which results in a finite delay between the time the enabling input signal is provided and the time the amplifier is able to provide the output signal. For example, in a fluid mass flow controller of the kind described above, the amplifier includes an associated feedback network. This circuit is characterized by a delay, or response, time which must elapse between the time the enabling input signal arrives and the time the amplifier is able to provide the valve signal which causes the valve to open and initiate the flow.
Although the response time which is characteristic of an amplifier having reactive control loop components may be of little significance if the actual time at which the process commences is not critical, it is of great importance in those applications in which the moment at which the process commences must be accurately controlled. This is especially true when only a relatively small change in the magnitude of the process is desired because it often takes longer for the control loop to respond to a small input signal than to a large one. Accordingly, there is a need for a way to minimize this response time in electronic control circuits.